This disclosure relates generally to diagnostic imaging and, more particularly, to an apparatus and method of high resolution magnetic resonance spectroscopic imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (such as a polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subject to a magnetic field (excitation field B1) in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method used. The resulting set of received signals are digitized and processed to reconstruct the image using one of many known reconstruction techniques.
Magnetic resonance spectroscopic imaging (MRSI) is a noninvasive imaging method that provides spectral information in addition to the structural information that is generated by magnetic resonance imaging (MRI) alone. Traditional MRI generates a black-and-white image in which brightness is determined primarily by the water molecule concentrations, and the T1 and/or T2 relaxation times of the tissue being imaged; the spectral information obtained in MRSI provides additional information about metabolic activity. MRSI can be performed on a standard MRI scanner, and has broad applications in medicine, oncology, and general physiological studies. And, when hydrogen is the target element, MRSI is also called 1H-nuclear magnetic resonance spectroscopic imaging and proton magnetic resonance spectroscopic imaging. Similarly, we have 31P magnetic resonance spectroscopic imaging and 13C magnetic resonance spectroscopic imaging.
MRSI has been recognized as a powerful tool for noninvasive metabolic studies, but clinical and research applications of this technology have been developing more slowly than expected. Reasons for the slow development of the technology include but are not limited to long data acquisition time, poor spatial resolution, and low signal-to-noise ratio (SNR), as examples.
Significant efforts have been made to address the above issues, resulting in a large number of new data acquisition and reconstruction methods for spectroscopic imaging. For fast data acquisition, one approach is to incorporate echo-planar-type of data acquisition schemes with spectroscopic imaging. Many methods (and pulse sequences) have been proposed to implement this data acquisition strategy. And, although echo-planar spectroscopic imaging (EPSI) methods can significantly reduce the data acquisition time for spatiospectral encoding, it is at the expense of SNR. Another approach to accelerated spectroscopic imaging is to use parallel imaging in which parallel data acquisition occurs using phased array coils.
Advanced MRSI reconstruction has focused on using prior information to compensate for the lack of sufficient measurements or SNR. To this end, a number of reconstruction models have been proposed, but reconstruction methods alone may not provide adequate levels of improvements in spatial resolution, data acquisition speed, and SNR needed to have a major impact on in vivo MR spectroscopic imaging.
As such, there is a need for improved MR spectroscopic imaging.